Cryogenic vacuum pumps (cryopumps) remove gases from a surrounding atmosphere by freezing gas molecules onto low-temperature cryopanels. Many recently-produced cryopumps follow a common design concept. One such cryopump was disclosed in U.S. Pat. No. 4,655,046, issued to Eacobacci and Planchard in 1987. An embodiment of this cryopump is illustrated in FIG. 1. The cryopump includes a housing 12 containing a two-stage cryogenic refrigerator 18 and at least two cryopanels including a primary pumping panel 34 and a radiation shield 44. The housing 12 has a flange 14 mounted to it at its open end.
When used in industry, the flange 14 is mounted to a port on a vessel which defines a work chamber. Through a front opening 16 of the cryopump, gas can travel from the work chamber, into a vacuum chamber defined by the housing 12. Within the vacuum chamber, gases are condensed on each of the cryopanels 34 and 44. The radiation shield 44 generally comprises a housing which is closed except at a frontal array 48 positioned between the primary pumping panel 34 and the chamber to be evacuated. The radiation shield 44 is cooled by a first stage 29 of the refrigerator 18 to a temperature in the range of 60 to 130 K. High-boiling-point gases, such as water vapor, which enter from the work chamber condense upon the frontal array 48, while the remainder of the radiation shield 44 serves primarily to shield the primary pumping panel 34 from radiant heat. The primary pumping panel 34 is typically maintained at 4 to 25 K by a second stage 32 of the refrigerator 18 and is used to condense lower-boiling-point gases which pass through the frontal array 48. The underside of the primary pumping panel 34 is coated with adsorbent charcoal 36 which can remove gases with especially-low boiling points, such as hydrogen. Other panels may, for example, include stacked plates having charcoal on the bottom surfaces of the plates.
The cryogenic refrigerator 18 in this embodiment is a two-stage refrigerator which achieves cooling through a Gifford-McMahon cooling cycle, wherein the refrigerator 18 extracts heat from the cryopanels 34 and 44 as it expands compressed helium gas. The refrigerator 18 is driven by a motor 22 and is supplied with helium through a feed line 24. Processed helium is removed from the refrigerator through a return line 26, which returns the helium to a compressor which recompresses the helium for repeated processing.
The cryopanels establish a vacuum within the vacuum chamber essentially by freezing gas molecules out of the atmosphere. When a free-floating gas molecule impacts a cryopanel, the cryopanel extracts thermal energy from the gas molecule. If enough thermal energy is extracted, the phase of the gas molecule will be transformed from a vapor to a solid condensate on the cryopanel. With the gases thus condensed and/or adsorbed onto the cryopanels, a high vacuum is created within both the vacuum chamber and the work chamber.
Once a high vacuum has been established, work pieces can be moved into and out of the work chamber through partially evacuated load locks. With each opening of the work chamber to a load lock, additional gases enter the work chamber. Those gases are then condensed onto the cryopanels to again evacuate the chamber and provide the necessary low pressures for processing. Over time, the efficiency and pumping capacity of the cryopump drop as the amount of condensate accumulated on the cryopanels increases. Moreover, a danger of damage to work pieces within the work chamber as well as a potential health and safety risk exists due to the potential of power outage or other causes of rapid warming which would cause the condensed gases, which may include hazardous chemicals, to sublimate.
Accordingly, the cryopanels 34 and 44 are periodically subjected to a regeneration procedure in which the cryopanels 34 and 44 are warmed under a controlled schedule to release the condensed gases from the cryopanels. The released gases are removed from the vacuum chamber through an exhaust conduit 58. At the end of the exhaust conduit 58 is a relief valve 60 which controls the flow of gas out of the vacuum chamber. The relief valve 60 can likewise provide an outlet for sublimating gases in the event of an unscheduled shutdown of the cryopump.
A typical relief valve 60 is a pressure-release valve which includes a cap, which, when the valve is closed, is held against an o-ring seal by a spring. If the pressure is sufficient to open the valve, the cap is pushed away from the o-ring seal and the exhausted gases flow past the seal. Absent a filter, debris entrained within the exiting gas stream also flows through the exhaust conduit and will often collect on the o-ring seal and closure cap. This debris includes particles of charcoal from the cryopanel or other debris resulting from processing within the work chamber. The accumulation of debris on the seal and cap prevents the valve from sealing. As a result, leaks into the cryopump develop at the relief valve, providing an undesired load on the cryopump.
In the embodiment illustrated in FIG. 1, a filter standpipe 62 is positioned at the mouth of the exhaust conduit to filter debris entrained within the exiting gas stream. The filter standpipe 62 includes a stainless steel mesh screen formed into a cylinder with an open end 64. The open end precludes a potentially dangerous pressure buildup in the chamber if the filter screen should otherwise become clogged. The filter standpipe 62 is at least about four inches in length and is installed in the cryopump through the front opening 16 either before the cryopanels 34 and 44 are installed or after removing the cryopanels 34 and 44 to provide access to the exhaust conduit 58 from within the vacuum chamber.